Problems in Aquatic Biology. 157 
Before proceeding to deal with the other features presented by 
the submerged growth it may be well to tabulate the vegetation of 
a piece of water as follows:— 
“A” Formation :—Semi-aquatic or marsh vegetation. 
(a) Cormophytic associations (e.g., Phragmitetum, Scirpetum) not 
always present. 
(b) Algal associations of the “ Spritzzone,” i.e., forms growing 
in well aerated and frequently disturbed water, often 
subjected to periodical exposure. 
“ B ” Formation : —Aquatic vegetation (type of formation dependent 
on air-content, etc., cf. p. 154). 
(a) Cormophytic associations. 
a. Free-floating (e.g., Lenina, Azolla, Hydro- 
charis, etc.) 
, (3. Attached with floating leaves 1 (e.g., Nuphar, 
Nupharetum, ' _ ^ v b 1 
„ Nymphaea, Potamogeton. 
Potamogetonetum, ■< 
. ‘ 7 . Submerged and attached 1 (e.g., Potamogeton, 
Cnaracetum. I , 
\ Mynophyllum , etc.) 
8. Submerged and not attached (e.g., Cerato- 
phyllum, Utricularia, etc.) 
(b) Algal associations. 
a. Plankton. 
(3. Submerged growth of photic region (perhaps 
to be united to a 8). 
y. Submerged growth of aphotic region. 
In every piece of water, apart from noticing these different 
associations, we find examples of more intimate relationships, which 
I will briefly touch upon now. I have already pointed out above 
that Cladophoraceae generally bear a growth of characteristic 
epiphytes and I have no doubt that more detailed investigation than 
has as yet been possible will show that many filamentous Algae bear 
epiphytes, more or less restricted to them. The reasons for these 
restrictions are probably due to a diversity of conditions, presented 
by the character and mode of life of the Algae bearing the epi¬ 
phytes, e.g., structure of the wall (with or without a thick mucilaginous 
investment, etc.), position in the water (near the surface or deeply 
submerged, in a shaded or exposed position), assimilatory and 
respiratory activity and consequent amount of excreted oxygen and 
carbon dioxide, period of maximum abundance, etc. There are 
1 It is possible that /3 and y are not distinct associations, but the 
evidence for their union as Nupharetum, Potamogetonetum, 
etc. is not quite satisfactory. 
